The present application claims priority to Japanese Application No. P11-097896 filed Apr. 5, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The invention relates to a thin film semiconductor device such as a thin film solar battery made of a silicon thin film, a semiconductor such as CIS, or an organic material such as a conductive polymer and a method of manufacturing the same.
2. Description of the Related Art
Recently, solar batteries have been put to practical use in some field. Resource saving and cost reduction are important in order that the solar battery may be widely and generally used. Moreover, a thin film solar battery is more desirable than a thick film solar battery for an improvement in energy conversion (photoelectric conversion) efficiency, a reduction in the number of years for energy recovery, and so on. Furthermore, the thin film solar battery has an advantage of having a wide range of applications. That is because the thin film solar battery can be bent to some extent and thus it can be mounted on, for example, a curved surface of a body of an automobile or an outer curved surface of a portable electrical appliance to generate electric power.
The applicant of the invention has previously proposed the following method as a preferred method for manufacturing a thin film solar battery (Japanese Patent Laid-open Nos. Hei 8-213645 and Hei 10-135500). The method includes: forming a porous layer as an isolation layer on a single crystal silicon substrate; growing a semiconductor layer made of thin film single crystal silicon for functioning as a solar battery on the porous layer; then bonding a thin plastic plate to the semiconductor layer with an adhesive; and then peeling the semiconductor layer together with the plastic plate from the single crystal silicon substrate.
On the other hand, a technique for improving conversion efficiency of such a thin film solar battery is that a texture structure having fine protrusions is made on a silicon substrate and a solar cell is made on the texture structure, whereby light is confined within the solar battery. However, when the fine protrusions are formed on the surface of the substrate and the solar battery is formed on the fine protrusions, a problem exists: a short-circuit current value does not increase much in spite of a considerable reduction in reflectance on the surface of the substrate. A further improvement is therefore desired.
Moreover, there is a method for improving efficiency of a solar battery. In the method, sunlight is scattered by a texture structure made on a rear surface of a thin film solar battery, whereby part of the light is emitted again from the solar battery, part of the light is totally reflected by the surface of the solar battery and the reflected light is again scattered by the texture structure on a bottom surface. Thus, a distance of transmission of the light through the solar battery is increased, and consequently the efficiency of the solar battery is improved. However, the method has another problem: a direction of light scattering is irregular because of a random texture structure and therefore it is substantially impossible to eliminate the light emitting from the solar battery.
A solar battery having another structure is also manufactured. The structure has a reflecting mirror which is made by coating the bottom surface of the solar battery with a reflecting material such as aluminum (Al), wherein the light passing through the solar battery is returned to the solar battery by the reflecting mirror. However, the structure has still another problem. That is, most of the light that is reflected and thus returned to the solar battery, except the light reflected by the surface of the solar battery, again emits from the solar battery into the air, and therefore the conversion efficiency cannot be sufficiently increased.
On the other hand, a single crystal silicon thin film solar battery not using a silicon substrate can save a silicon material, and thus this solar battery is important from the viewpoint of manufacturing costs and resource saving. However, because of a thin silicon film, a considerable amount of incident light is not absorbed by the silicon film but passes through the silicon film. Thus, the single crystal silicon thin film solar battery has a further problem of being unable to obtain high efficiency of electric power generation. For example, when the light has a wavelength of 800 nm and the silicon thin film has a thickness of 10 xcexcm, silicon has the optical absorption efficiency of 67%. Optical absorption of silicon is maximized at a wavelength of about 200 nm, while the optical absorption monotonously decreases with increasing wavelength. Accordingly, the light with wavelengths longer than 800 nm substantially passes through the silicon thin film, and thus the photoelectric conversion efficiency decreases.
The invention is designed to overcome the foregoing problems. It is an object of the invention to provide a thin film semiconductor device and a method of manufacturing the same capable of improving the optical absorption efficiency of a single crystal silicon thin film or the like and thus improving the photoelectric conversion efficiency.
A thin film semiconductor device of the invention comprises: a photoelectric conversion layer for converting incident light into an electric signal; and a diffraction function layer located on one side of the photoelectric conversion layer opposite to the other side on which the light is incident, for reflecting the light passing through the photoelectric conversion layer toward the photoelectric conversion layer.
A method of manufacturing a thin film semiconductor device of the invention comprises the steps of: pressing a master of the diffraction function layer on a transparent thermoplastic resin layer or a transparent thermosetting resin layer and thereby transferring a grating shape to the resin layer, and then filling a light reflecting material into a transfer surface of the resin layer, thereby forming a reflection-type diffraction function layer; and bonding the diffraction function layer to one surface of the photoelectric conversion layer opposite to the other surface on which the light is incident.
In the thin film semiconductor device of the invention, the light passing through the photoelectric conversion layer is reflected toward the photoelectric conversion layer by the diffraction function layer, and the reflected light again enters into the photoelectric conversion layer. A condition under which the reflected light is totally reflected toward the photoelectric conversion layer is set, whereby the light is confined within the photoelectric conversion layer and thus the photoelectric conversion efficiency is improved.
In the method of manufacturing a thin film semiconductor device of the invention, the master of the diffraction function layer is pressed on transparent thermoplastic resin or transparent thermosetting resin. The grating shape is transferred to the resin layer. Then, the light reflecting material such as aluminum (Al) is filled into the transfer surface of the resin layer, whereby the reflection-type diffraction function layer is formed. The reflection-type diffraction function layer is located on one surface of the photoelectric conversion layer opposite to the other surface on which the light is incident, with the resin layer sandwiched therebetween.
Other and further objects, features and advantages of the invention will appear more fully from the following description.